The purpose of this study is to assess circadian functioning in people with Parkinson's disease by determining the dim-light melatonin onset and endogenous melatonin secretion level. Subjects will be admitted to the GCRC at @4:00 p.m., so that 30-minute blood sampling can be begun at 6:00 p.m. and continued until 8:00 a.m. the following morning. A variety of other procedures and questionnaires will be administered by study staff, and sleep polysomnography will be employed by a sleep technician. Subjects will return to the GCRC for a second night of polysomnography the following day, but no blood draws or additional procedures will occur.